Oxygen-peracetic acid-chlorine dioxide pulp bleaching sequence

ABSTRACT

A PROCESS OF BLEACHING CHEMICAL PULPS WHICH COMPRISES A FIRST STAGE OF BLEACHING WITH OXYGEN IN THE PRESENCE OF NA2S2O4 OR NA2SX, WHEREIN X IS AN INTEGER FROM 1 TO 4, AND OPTIONALLY NAOH, FOLLOWED BY A PERACETIC ACID SECOND STAGE, RESULTS IN A PULP HAVING A BRIGHTNESS OF ABOUT 60% OR ABOVE WHICH IS SUITABLE FOR NEWSPRINT FURNISH. A FURTHER BLEACHING STAGE EMPLOYING CHLORINE DIOXIDE RESULTS IN A PULP HAVING A BRIGHTNESS OF ABOUT 80% OR ABOVE WHICH IS SUITABLE FOR DISSOLVING PULP AND WRITING AND BOND PAPERS.

United States Patent 3,695,995 OXYGEN-PERACETIC ACID-CHLORINE DIOXIDE PULP BLEACHING SEQUENCE Sunanda K. Royrnoulilr, Sulfern, N.Y., assignor to international laper Company, New York, N.Y.

No Drawin Filed lylay 25, 1970, Ser. No. 40,404 Int. Cl. DZlc 9/10, 9/12, 9/16 U.S. Cl. 162-55 16 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a novel 2 or 3 stage sequence for the delignification and bleaching of chemical pulps.

Most commercial bleaching sequences used today employ 5 or 6 stages. When bleaching chemical pulps, either sulfate (kraft) or sulfite, one or more of the stages employs elemental chlorine. Since chlorination is normally carried out at a consistency of 3% in the first stage of a conventional bleaching sequence, based upon the weight of the pulp, extremely large volumes of water are required for the bleaching system. This results in a large amount of chlorine containing efiiuent, i.e., chloride ions, being discharged into and polluting the streams adjacent the bleachery.

When elemental chlorine is used in the first stage of most kraft bleaching sequences, e.g., CEDED or CEHDE-D, its purpose is to form alkali-soluble chlorolignins which can be extracted in the second stage by caustic soda. i.e., E stage. However, the use of chlorine in the first stage degrades the cellulosic portion of the pulp. This means that there is a loss in the degree of polymerization, pulp strength is diminished, and the yield is reduced.

Accordingly, it is an object of the present invention to provide a process for bleaching chemical pulps while minimizing the generation of chloride ions in the efiluent, to help abate stream pollution, and to minimize cellulose degradation.

It is another object of the present invention to provide a novel two-stage pulp bleaching sequence to obtain a pulp having a brightness of about 60% or above.

It is still another object of the present invention to provide a novel three-stage pulp bleaching sequence to obtain a pulp having a brightness in excess of 80%.

It has been found that when a chemical pulp is bleached in the first stage with oxygen in the presence of a compound selected from the group consisting of Na S O and Na s and, optionally, sodium hydroxide, followed by a second stage employing peractic acid, a pulp having a brightness of about 60% or above is obtained. If the second stage, i.e., peracetic acid bleach stage, is followed by a third stage employing chlorine dioxide, either gaseous or in solution, the resulting pulp has a brightness in excess of 80%.

In the first stage an unrefined, digested chemical pulp, prepared by either the sulfate or sulfite process, and having a consistency of from about 3% to 35% and pref- 3,695,995 Patented Oct. 3, 1972 erably above 20%, based on the weight of the oven-dried pulp, is contacted or reacted with oxygen, at a temperature of from about 70 C. to about 140 C., preferably about 100 C., and a pressure of from about 60 psi. to about 130 p.s.i., preferably about 100 p.s.i., for approximately one-half hour to about eight hours in the presence of at least 0.2%, based on the weight of the pulp, of a protector compound selected from the group consisting of polysulfides having the formula Na S wherein x is an integer from 1 to 4, and Na S O Throughout this application the use of the term protector compound shall mean either an oxidizing or reducing agent which is capable of preventing the degradation and depolymerization of the cellulosic portion of the pulp by reacting with the carbonyl end groups, to convert them into hydroxyl groups when Na S O a reducing agent is used, or carboxyl groups, when Na s an oxidizing agent is used. Na S is the preferred polysulfide.

The concentration of Na S O or Na S should be at least about 0.2% and preferably from about 2% to about 10%, based on the weight of the oven-dry pulp. If concentrations in excess of 10% are employed, costs are increased and no additional benefits are obtained.

It should be noted that since the protectors contain only sodium and sulfur ions, both of which are present in sulfate or sulfite pulping systems, the efiiuent can be fed back into the chemical recovery system thereby preventing stream pollution which would otherwise occur if the efiiuent were emptied into the stream, as is conventionally done.

If pulps having a lignin content of 5% or above are employed, the addition of from about 1% to about 5% of sodium hydroxide to the reaction mixture can speed up the reaction rate. Also, if the concentration of Na S O or Na S is 2% or less, sodium hydroxide can be employed to good advantage.

After water washing followed by centrifugation 01' filtration, the partially bleached pulp of the first stage is subjected in the second stage to a bleach with peracetic acid. The concentration of the peracetic acid can be from about 0.5% to about 2.0%, preferably 1.0%, based on the weight of the oven-dried pulp, and the pulp consistency can be from about 10% to about 20%, preferably between about 10% and about 15%. The reaction can be carried out at a temperature of from about 50 C. to about 90 C., with 70 C. being preferred. During the peracetic acid second stage, which can take from about 30 minutes to 120 minutes, the initial or starting pH must be within the range of from about 8.0 to about 12.0, with a range of from about 8.0 to about 10.0 being preferred. The pH can be adjusted by adding Na CO and/or NaOH. The initial pH must be adjusted to 8.0 or above to optimize the bleaching results.

The pulp after the first and second stages, namely, oxygen (0) and peracetic acid (Pa), has a brightness of 60% or above and can be utilized as newsprint furnish.

In order to improve pulp brightness to or above, so that it can be used as a dissolving pulp or in the manufacture of bond papers, the pulp is treated in a third stage with chlorine dioxide. Either gaseous chloride dioxide or chlorine dioxide in solution can be employed.

When a solution of chlorine dioxide in water is used, the consistency of the pulp can be from about 10% to about 20%, preferably between 10% and 15%, based on the weight of the oven-dried pulp. The concentration of the C10 can be from about 0.5% to about 2.0%, based on the weight of the oven-dried pulp. The reaction can be carried out at a temperature of from about 50 C. to about C., preferably 70 C., and at a starting pH be tween 5.0 and 6.0 for a period of from about 30 minutes to about minutes.

When gaseous chlorine dioxide is used, the pulp consistency can be from about 10% to about 40%, with a consistency above 30% being preferred. The reaction time can be reduced to about to minutes and can take place at a temperature of from about 20 C. to above 5 90 C., with 70 C. being preferred.

The following examples illustrate the process of the Reaction temperature 70 C. Reaction time 2 hours.

The semi-bleached pulp was then thoroughly washed with water and centrifuged. The brightness of the pulp was found to be 64% as measured in accordance with Tappi Standard Method T217.

present invention. 3rd Stage EXAMPLE 1 Chlorine dioxide 10.0%.

10 5.5-6.0. A 100 gram sample of a hardwood pulp, which had ggfi ig g been prepared in accordance with the kraft process and Reaction temperature 2 hours. which had then been washed with deionized water, was Reaction time 05% on 0D Pulp bleached as follows. (The first stage was carried out in an electrically heated pressure reactor.) The fully bleached pulp was then washed with water and centrifuged. The results of its evaluation are listed Stage below in Table 1. Reaction temperature 100 C. The properties enumerated in Table 1 below were de- Reaction pressure (0 100 p.s.i. termined in accordance with Tappi Standard Test Meth- Reaction time 3 hours. ods. They are, as follows: Brightness, T217; CSF, T227; Pulp consistency 20-22%. Burst Factor, T220 and T403; Tear Factor, T220 and Na S O (Hydrosulfite) 2% on 0D. pulp. T414; Breaking Length, T220 and T404; ASV, T220 and NaOH 2% on DD. pulp. Opacity, T425.

TABLE 1 Bright Burst Tear Breaking Opacness CSF factor factor length ASV 1ty Examplel 85.3 372 44.3 91.0 8,056 1.50 78.0 Example2 84.1 390 47.7 73.4 8, 275 1.44 75.0 ooEDED (conventional)--- 85.5 395 43.0 84.0 7, 435 1.53 65.0

The pulp was then thoroughly washed with water, cen- 3 trifuged and further bleached in accordance with stage 2 below.

2nd Stage Peracetic acid 1% on 0D. pulp. Pulp consistency 10%. 3 Starting pH (adjusted with NaOH) 8.0. Reaction temperature 70 C. Reaction time 2 hours.

The semi-bleached pulp was thoroughly washed with water and centrifuged. The brightness of the pulp was found to be 66% as measured in accordance with Tappi Standard Method T217.

3rd Stage Chlorine dioxide 0.5% on 0.1). pulp. Pulp consistency 10%. Starting pH 5.5-6.0. Reaction temperature 70 C. Reaction time 2 hours.

i The fully bleached pulp was then washed with water centrifuged and evaluated. The results of the evaluation are listed below in Table 1.

EXAMPLE 2 A 100 grams sample of a hardwood pulp, which had been prepared in accordance with the kraft process and which had then been washed with deionized water, was bleached as follows. (The first stage was carried out in an electrically heated pressure reactor.)

After thorough washing and centrifugation, the plup was further bleached in accordance with stage 2 below.

2nd Stage Peracetic acid 1% on DD. pulp.

Pulp consistency 10%.

Starting pH (adjusted with NaOH) 8.0.

It can readily be seen from Table 1 that the O-Pa-D sequence of the present invention (Examples 1 and 2) results in a fully bleached pulp of a brightness comparable to that obtained by a S-stage bleaching sequence and having a higher opacity. The physical properties are equiva- 5 lent to those obtained by a conventional bleaching sequence.

What is claimed is:

1. A two-stage process for bleaching chemical pulps to a brightness of 60% or above, which comprises:

(a) a first stage of reacting said pulp with oxygen in the presence of a protector whose concentration is from about 0.2% to about 10%, based on the ovendried weight of the pulp, said protector being selected from the group consisting of Na S O and Na S wherein x is an integer from 1 to 4, at a pressure of from about 60 p.s.i. to about 130 p.s.i. and a temperature of from about 70 C. to about 140 C., for from about V2 hour to about 8 hours;

(b) a second stage of reacting the pulp with from about 0.5% to about 2.0% of peracetic acid, based on the weight of the oven-dried pulp, at a temperature of from about 50 C. to about C. for a period of from about 30 minutes to about minutes.

2. The process as recited in claim 1 wherein the starting pH of the peracetic acid second stage is from about 8 to about 12.

3. The process as recited in claim 1 wherein the starting pH of the peracetic acid second stage is from about 8 to about 10.

4. The process as recited in claim 1 wherein the consistency of the pulp is from about 3% to about 35%.

5. The process as recited in claim 1 wherein the oxygenation first sage is carried out m the presence of from about 0.5 to about 5% of sodium hydroxide, based on the Weight of the pump, and the concentration of the protector is 2% or less, based on the weight of the ovendried pulp.

6. The process as recited in claim 1 wherein the protector in the first stage is Na S and its concentration is from about 2% to about 10%, based on the weight of the oven-dried pulp.

7. The process as recited in claim 1 wherein the protector in the first stage is Na S O and its concentration is from about 2% to about 10%, based on the weight of the oven-dried pulp.

8. A three-stage process for bleaching chemical pulps to a brightness of 80% or above, which comprises:

(a) a first stage of reacting said pulp with oxygen in the presence of a protector Whose concentration is from about 0.2% to about 10%, based on the oven-dried weight of the pulp, said protector being selected from the group consisting of Na S O and Na S wherein x is an integer from 1 to 4, at a pressure of from about 60 psi. to about 130 psi. and a temperature of from about 70 C. to about 140 C. for from about /2 hour to about 8 hours;

(b) a second stage of reacting the pulp with from about 0.5% to about 2.0% of peracetic acid, based on the weight of the oven-dried pulp, at a temperature of from about 50 C. to about 90 C. for a period of from about 30 minutes to about 120 minutes;

() a third stage of reacting the pulp with from about 0.25% to about 2.0% of chlorine dioxide, based on the weight of the oven-dried pulp, at a temperature of from about 20 C. to about 90 C. for a period of from about minutes to about 120 minutes.

9. The process as recited in claim 8 wherein the starting pH of the peracetic acid second stage is from about 8 to about 12.

10. The process as recited in claim 8 wherein the starting pH of the peracetic acid second stage is from about 8 to about 11. The process as recited in claim 8 wherein the consistency of the pulp is from about 3% to about 35%.

12. The process as recited in claim 8 wherein the oxygenation first stage is carried out in the presence of from about 0.5% to about 5% of sodium hydroxide, based on the weight of the pulp, and the concentration of the protector is 2% or less, based on the weight of the oven-dried pulp.

13. The process as recited in claim 8 wherein the protector in the first stage is Na S and its concentration is from about 2% to about 10%, based on the Weight of the oven-dried pulp.

14. The process as recited in claim 8 wherein the protector in the first stage is Na S O and its concentration is from about 2% to about 10%, based on the weight of the oven-dried pulp.

15. The process as recited in claim 8 wherein the chlorine dioxide in the third stage is chlorine dioxide in solution with a consistency of from about 10% to about 20%, based on the weight of the oven-dried pulp, and the temperature is about 70 C. for a period of from about 30 to about 120 minutes.

16. The process as recited in claim 8 wherein the chlorine dioxide in the third stage is gaseous chlorine dioxide with a consistency of from about 10% to about based on the weight of the oven-dried pulp, and the reaction temperature is about C. for a period of from about 5 to about 10 minutes.

References Cited UNITED STATES PATENTS 3,423,282 1/1969 Rerolle et a1. 162-65 3,085,039 4/ 1963 Gartner 162-78 3,384,533 5/1968 Robert et al. 162-65 2,442,332 6/1948 Haney et a1 162-76 X 3,458,394 7/1969 Yiannos et a1. 162-76 3,536,577 10/ 1970 Rapson 162-78 X FOREIGN PATENTS 56,526 1922 Sweden 162-83 S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner U.S. C]. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 95,995 Dated October R, 1972 Inventor(s) Sunande 1'1. Roymoulik It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In th specification, column l, line 9 delete "10.0%",

, inse t "0.5;; on O.D. pulp"; column 4, line 10., delete 5.5-6.0" insert "1055; column 4; line ll, delete "70 ineer't 5.56.0-; column I, line 12, delete "2 hours", in ert -70C column l, line 13, delete "0.5% on (.D. nuln", 'inwrt "2 hour s--.

n the Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

EDWARD M.PLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents claims, claim 5, line 4, delete "pump", insert -pulp.

ORM PO-105O (IO-69) USCOMM-DC 60370-5 60 uls. eovununrr nnnmc OVHCI: In! o-su-ul UNITED STATES PATENT OFFICE I CERTIFICATE OF CORRECTION Patent No. 3, 95,995 Dated October BL 1972 Inventor(s) Sunande K. ROymOulik It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Tn the specification, column I, line 9, delete "10.075", insert O.5';$ on 0.13. pulp",- column line 10, delete "5.5-6.0

insert --lO,;- column L.- line 11, delete "70C.", insert -5.E}-6.0-; column L, line 12, delete "2 hours", insert --70C.-: column 4, line 13, delete "0.5% on (.D. pulp", in wr't hOUIS--.

In the claims, claim 5, line delete "pump", insert --pulp-- Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT'GOTTSCHALK Attestlng Officer Commissioner of Patents "ORM PO-1050 (10-69) USCOMM-DC B0376-P69 ".5. GOVIINIINT PIIITIIIG OFIICI "O! O-IOPJJO 

